An interesting new research project out of Sweden's Linköping University has demonstrated how plant roots can be used as energy storage devices. The scientists see the process, which requires the plants to be watered with a special solution to make their roots electrically conductive, as as proof-of-concept for a root-based supercapacitor, as well as for biohybrid systems that meld biological processes with electronic functionality.

Carried out at the university's Laboratory of Organic Electronics, the breakthrough builds on previous research led by Dr Eleni Stavrinidou's Electronic Plants Group. In 2015, these scientists were able to fabricate electrical circuits in the vascular tissue of roses by dosing the plants with a conductive polymer called PEDOT, with the circuits then used to form transistors. In 2017, the scientists added a conjugated oligomer called ETE-S instead, which formed polymers within the plant that turned into electrical conductors capable of storing energy.

“We have previously worked with plants cuttings, which were able to take up and organize conducting polymers or oligomers," says Stavrinidou. "However, the plant cuttings can survive for only a few days, and the plant is not growing anymore. In this new study we use intact plants, a common bean plant grown from seed, and we show that the plants become electrically conducting when they are watered with a solution that contains oligomers."

The bean plant used in the team's experiments is called Phaseolus vulgaris, and polymerizes the conjugated oligomer ETE-S contained in the watering solution as part of a natural process. This sees a conductive film of polymer form on its roots, and turns its entire root system into a network of conductors, which remained electrically active for more than four weeks.

The scientists adapted this for use as supercapacitor, with the roots acting as the system's electrodes during charging and discharging. They found that it could store 100 times the energy of its previous systems that used only the plant stems, and that this appeared to have little effect on the wellbeing of the plant itself, enabling the system to be used over extended periods of time.

“The plant develops a more complex root system, but is otherwise not affected: it continues to grow and produce beans,” says Stavrinidou.

In the view of the researchers, the work presents a promising pathway for integrating energy systems into living plants without compromising their biological functions. We have seen a number of interesting research projects centering on similar biohybrid systems, some of which aim to turn plants into organic surveillance sensors and others that involve cyborg plants that can drive themselves toward light. The development of electronic roots by the Linköping University scientists could serve as a useful addition to this area of study.

The research was published in the journal Materials Horizons.